Polyacetal resin gear and method for improving fatigue resistance of polyacetal resin gear

Abstract

To provide a polyacetal resin-made gear which is excellent in fatigue resistance characteristics.SOLUTION: A polyacetal resin-made gear is formed of a polyacetal resin or a resin composition containing a polyacetal resin, wherein the polyacetal resin is obtained by copolymerization of trioxane (A), a cyclic acetal compound (B) and an aliphatic glycidyl ether compound (C) having one glycidyloxy group in one molecule, and in the whole components (A) to (C), the cyclic acetal compound (B) is 0.5-2.5 mol%, and the aliphatic glycidyl ether compound (C) is 0.02-0.7 mol%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a gear made of polyacetal resin and a method for improving the fatigue resistance of a gear made of polyacetal resin. [Background technology]

[0002] Polyacetal resin (also known as polyoxymethylene resin, abbreviated as POM resin) has well-balanced mechanical properties and is excellent in friction and wear resistance, chemical resistance, heat resistance, electrical properties, etc., and is therefore widely used in fields such as automobiles and electrical and electronic products. In particular, POM resin is useful as a gear material, and gears made of POM resin are widely used (see Patent Document 1).

[0003] One of the performance requirements for gears is a long period of time before failure occurs during operation (hereinafter referred to as "fatigue resistance"). POM resins are broadly divided into two types: homopolymers, which have only oxymethylene units (-CHO-) in their unit structure, and copolymers, which have oxymethylene units as well as oxyethylene units (-CHCHO-). Of these two types of POM resins, copolymers have superior fatigue resistance and are more suitable for gear applications, considering that gears will be operated for long periods of time. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-70146 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, polyacetal copolymers are suitable for gear applications, and if the fatigue resistance of polyacetal copolymers can be further improved, their usefulness for gear applications will be further enhanced.

[0006] The present invention has been made in consideration of the above-mentioned problems of the prior art, and an object of the present invention is to provide a polyacetal resin gear having excellent fatigue resistance, and a method for improving the fatigue resistance of a polyacetal resin gear. [Means for solving the problem]

[0007] As a result of extensive research to solve the above problems, the present inventors have found that polyacetal copolymers obtained by polymerization using specific monomer species have excellent fatigue resistance, and have thus completed the present invention.

[0008] One aspect of the present invention that solves the above problem is as follows. (1) A polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin, the polyacetal resin is obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule; A polyacetal resin gear, wherein the cyclic acetal compound (B) is 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) is 0.02 to 0.7 mol % in the entire components (A) to (C).

[0009] (2) The polyacetal resin gear according to (1) above, wherein the polyacetal resin or the resin composition has a melt flow rate of 0.5 to 3.0 g / 10 min.

[0010] (3) The polyacetal resin gear according to (1) or (2) above, wherein the aliphatic glycidyl ether compound (C) is a compound represented by the following general formula (1):

[0011] [ka] [R represents a hydrocarbon group having 3 to 10 carbon atoms.]

[0012] (4) The polyacetal resin gear according to any one of (1) to (3) above, wherein the aliphatic glycidyl ether compound (C) is butyl glycidyl ether or 2-ethylhexyl glycidyl ether.

[0013] (5) A method for improving fatigue resistance of a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin, comprising: A method for improving the fatigue resistance of a gear made of polyacetal resin, using as the polyacetal resin a polyacetal resin obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule, wherein the cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % of the total of the (A) to (C) components. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a polyacetal resin gear having excellent fatigue resistance, and a method for improving the fatigue resistance of a polyacetal resin gear. DETAILED DESCRIPTION OF THE INVENTION

[0015] <Polyacetal resin gear> The polyacetal gear of this embodiment is a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin. The polyacetal resin is obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule (hereinafter simply referred to as "aliphatic glycidyl ether compound (C)"). The cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % of the total components (A) to (C). In this specification, "polyacetal resin (POM resin)" is also called polyacetal copolymer, since it is classified as a copolymer when it comes to whether it is a homopolymer or a copolymer.

[0016] The POM resin according to this embodiment is believed to have a structure composed of structural units derived from trioxane (A), structural units derived from a cyclic acetal compound (B), and oxyethylene groups to which alkoxymethyl groups derived from an aliphatic glycidyl ether compound (C) are bonded as substituents. Although the specific structure is unknown, the polyacetal copolymer obtained by copolymerization as described above has excellent fatigue resistance.

[0017] Each component will be described below.

[0018] [Trioxane (A)] Trioxane (A) is a cyclic trimer of formaldehyde, and is generally obtained by reacting an aqueous formaldehyde solution in the presence of an acidic catalyst, and is used after being purified by a method such as distillation. The trioxane (A) used in polymerization is preferably one in which impurities such as water and methanol have been reduced as much as possible.

[0019] [Cyclic acetal compound (B)] The cyclic acetal compound (B) is a cyclic acetal compound (B) copolymerizable with trioxane (A), and examples thereof include 1,3-dioxolane, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, and 1,6-hexanediol formal, with 1,3-dioxolane being preferred.

[0020] The copolymerization amount of the cyclic acetal compound (B) is 0.5 to 2.5 mol%, preferably 0.5 to 2.3 mol%, and more preferably 1.5 to 2.2 mol% of the total components consisting of trioxane (A), the cyclic acetal compound (B), and the aliphatic glycidyl ether compound (C). If the copolymerization ratio of the cyclic acetal compound (B) is less than 0.5 mol%, the stability when used with acidic grease decreases, and if it exceeds 2.5 mol%, the fatigue resistance becomes poor.

[0021] [Aliphatic glycidyl ether compounds (C)] The aliphatic glycidyl ether compound (C) is a general term for an aliphatic organic compound having one glycidyloxy group in the molecule. In this respect, it is distinguished from the cyclic acetal compound (B). As such an aliphatic glycidyl ether compound (C), a monofunctional glycidyl ether compound having one glycidyloxy group can be used.

[0022] Specific examples of the monofunctional glycidyl ether compound include methyl glycidyl ether, ethyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, 2-methyloctyl glycidyl ether, etc. Preferred are butyl glycidyl ether and 2-ethylhexyl glycidyl ether.

[0023] The aliphatic glycidyl ether compound (C) is preferably a compound represented by the following general formula (1): In the following general formula (1), when the hydrocarbon group represented by R has 3 to 10 carbon atoms, the fatigue resistance of the POM resin can be improved without impairing the basic properties of the POM resin.

[0024] [ka] [R represents a hydrocarbon group having 3 to 10 carbon atoms.]

[0025] In general formula (1), R represents a hydrocarbon group having 3 to 10 carbon atoms, preferably 3 to 9 carbon atoms. The hydrocarbon group represented by R may be linear or branched, or may form a ring. The hydrocarbon group represented by R may have an unsaturated bond. Specific examples of the hydrocarbon group include alkyl groups such as propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, and n-decyl.

[0026] More specifically, the aliphatic glycidyl ether compound (C) is preferably butyl glycidyl ether or 2-ethylhexyl glycidyl ether.

[0027] The copolymerization amount of the aliphatic glycidyl ether compound (C) is 0.02 to 0.7 mol %, preferably 0.03 to 0.5 mol %, and particularly preferably 0.05 to 0.3 mol %, based on the total amount of components (A) to (C). If the copolymerization amount of component (C) is less than 0.02 mol %, the effect of improving fatigue resistance cannot be obtained. Conversely, if it exceeds 0.7 mol %, problems such as poor moldability due to reduced fluidity may occur, and further, the crystallinity of the resulting copolymer may decrease, resulting in reduced fatigue resistance and rigidity.

[0028] In this embodiment, from the viewpoint of fatigue resistance and rigidity, it is particularly preferable to use one or more compounds selected from n-butyl glycidyl ether and 2-ethylhexyl glycidyl ether as the aliphatic glycidyl ether compound (C).

[0029] The molecular weight of the aliphatic glycidyl ether compound (C) is preferably 100 to 220. If the molecular weight of the aliphatic glycidyl ether compound (C) exceeds 220, the crystallinity of the POM resin obtained by copolymerization may be disturbed, impairing its basic properties and causing undesirable effects on fatigue resistance and rigidity. Conversely, if the molecular weight of component (C) is less than 100, the effect on fatigue resistance and rigidity will be extremely small.

[0030] In this embodiment, the polyacetal copolymer is basically obtained by bulk polymerization of trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) using a cationic polymerization catalyst, with an appropriate amount of a molecular weight modifier added as needed.

[0031] In this embodiment, to obtain a polyacetal copolymer having excellent thermal stability, rigidity, impact resistance, etc., it is preferable that the structural units derived from the cyclic acetal compound (B) and the aliphatic glycidyl ether compound (C) are uniformly dispersed in the molecular chain of the polyacetal copolymer. To achieve this, when producing the polyacetal copolymer by polymerization, it is effective to uniformly mix the cyclic acetal compound (B) and the catalyst, add this to a homogeneous mixture of the aliphatic glycidyl ether compound (C) and trioxane (A) that has been homogeneously mixed in advance, and then supply the mixture to a polymerizer for polymerization. By mixing the components in advance to form a homogeneous solution, the structural units derived from the cyclic acetal compound (B) and the aliphatic glycidyl ether compound (C) are well dispersed, resulting in improved mechanical properties and excellent thermal stability.

[0032] When producing the polyacetal copolymer of this embodiment comprising the above-mentioned constituent components, the polymerization apparatus is not particularly limited, and a known apparatus can be used, and either a batch method, a continuous method, or the like is possible. The polymerization temperature is preferably maintained at 65 to 135°C. The deactivation after polymerization is carried out by adding a basic compound or an aqueous solution thereof to the reaction product discharged from the polymerization reactor after the polymerization reaction or to the reaction product in the polymerization reactor.

[0033] Examples of the cationic polymerization catalyst used in this embodiment include lead tetrachloride, tin tetrachloride, titanium tetrachloride, aluminum trichloride, zinc chloride, vanadium trichloride, antimony trichloride, phosphorus pentafluoride, antimony pentafluoride, boron trifluoride, boron trifluoride diethyl etherate, boron trifluoride dibutyl etherate, boron trifluoride dioxanate, boron trifluoride acetic anhydrate, boron trifluoride coordination compounds such as boron trifluoride triethylamine complex compounds, perchloric acid, acetyl perchlorate, Examples of suitable catalysts include inorganic and organic acids such as tetrachloroacetic acid, t-butyl perchlorate, hydroxyacetic acid, trichloroacetic acid, trifluoroacetic acid, and p-toluenesulfonic acid; complex salt compounds such as triethyloxonium tetrafluoroborate, triphenylmethylhexafluoroantimonate, allyldiazonium hexafluorophosphate, and allyldiazonium tetrafluoroborate; alkyl metal salts such as diethylzinc, triethylaluminum, and diethylaluminum chloride; heteropolyacids; and isopolyacids. Among these, boron trifluoride coordination compounds such as boron trifluoride, boron trifluoride diethyl etherate, boron trifluoride dibutyl etherate, boron trifluoride dioxanate, boron trifluoride acetic anhydrate, and boron trifluoride triethylamine complex compounds are particularly preferred. These catalysts can also be used by diluting them in advance with an organic solvent or the like.

[0034] The molecular weight modifier used in this embodiment is a linear formal compound. Examples of the linear formal compound include methylal, ethylal, dibutoxymethane, bis(methoxymethyl) ether, bis(ethoxymethyl) ether, and bis(butoxymethyl) ether. Among these, one or more compounds selected from the group consisting of methylal, ethylal, and dibutoxymethane are preferred.

[0035] The basic compound used to neutralize and deactivate the polymerization catalyst may be ammonia, an amine such as triethylamine, tributylamine, triethanolamine, or tributanolamine, or a hydroxide salt of an alkali metal or alkaline earth metal, or other known catalyst deactivators. After the polymerization reaction, it is preferable to quickly add an aqueous solution of such a compound to the product to deactivate it. After the polymerization and deactivation processes, washing, separation and recovery of unreacted monomers, drying, and other steps may be carried out as needed by conventional methods.

[0036] Furthermore, a stabilization treatment is carried out as necessary by a known method, such as decomposition and removal of unstable terminals or blocking of unstable terminals with a stabilizing substance, to obtain a POM resin. A POM resin composition may be prepared by blending various stabilizers and, as necessary, adding general additives for polyacetal resins, as long as the effects of the polyacetal resin gear of this embodiment are not impaired. The stabilizer used here may be one or more of hindered phenol compounds, nitrogen-containing compounds, hydroxides of alkali or alkaline earth metals, inorganic salts, carboxylates, and the like. Common additives include, for example, colorants such as dyes and pigments, lubricants, nucleating agents, release agents, weather stabilizers, antistatic agents, surfactants, organic polymer materials, inorganic or organic fibrous, powdery or plate-like fillers, etc., and one or more of these may be added.

[0037] In this embodiment, the melt flow rate of the POM resin or the resin composition containing the POM resin is preferably 0.5 to 3.0 g / 10 min, and more preferably 1.0 to 3.0 g / 10 min. When the melt flow rate is 0.5 to 3.0 g / 10 min, the fatigue resistance is excellent. The melt flow rate is a value measured in accordance with ISO 1133 under the conditions of a temperature of 190°C and a load of 2.16 kg.

[0038] In this embodiment, the copolymerization step is preferably set so that (b + c + d) / a = 1.5 to 7 μmol / g, where a is the total mass (g) of trioxane (A), cyclic acetal compound (B), and aliphatic glycidyl ether compound (C), b is the number of moles of the linear formal compound used as a molecular weight modifier, and c and d are the total moles of water and methanol contained in components (A), (B), and (C), respectively. Satisfying (b + c + d) / a = 1.5 to 7 μmol / g results in a polyacetal copolymer with a melt flow rate (MFR) of 0.5 to 3.0 g / 10 min, measured according to ISO 1133 at 190°C under a load of 2.16 kg. The water and methanol contained in (A), (B) and (C) are derived from the respective impurities.

[0039] The method for molding the polyacetal resin gear of this embodiment is not particularly limited, and various methods known in the art can be used. For example, the gear can be produced by feeding the above-mentioned polyacetal resin, various stabilizers, and additives into an extruder, melt-kneading them to form pellets of a POM resin composition, and then feeding the pellets into an injection molding machine equipped with a predetermined mold and injection-molding them.

[0040] Alternatively, the polyacetal resin gear of this embodiment can be formed by molding the above-described polyacetal resin composition into a plate or rod shape, and then subjecting it to a general molding process such as cutting.

[0041] The type of gear to which the polyacetal resin gear of this embodiment can be applied is not particularly limited, but examples include spur gears, racks, internal gears, helical gears, helical internal gears, helical racks, double helical gears, straight bevel gears, spiral bevel gears, zerol bevel gears, screw gears, cylindrical worm gears, etc.

[0042] <Method for improving fatigue resistance of polyacetal resin gears> The method for improving the fatigue resistance of a polyacetal resin gear of this embodiment is a method for improving the fatigue resistance of a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin. The polyacetal resin used is a polyacetal resin obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule, wherein the cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % of the total of components (A) to (C).

[0043] As explained above with respect to the polyacetal resin gear of this embodiment, a specific POM resin or a resin composition containing this POM resin can impart excellent fatigue resistance to the gear. Therefore, the fatigue resistance of a polyacetal resin gear can be improved by using this specific POM resin or a resin composition containing this POM resin. The POM resin used in the method for improving the fatigue resistance of a POM resin gear of this embodiment is as explained above with respect to the polyacetal resin gear of this embodiment, and the explanation and preferred aspects of the POM resin apply directly to the method for improving the fatigue resistance of a POM resin gear of this embodiment. [Example]

[0044] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to the following examples.

[0045] [Examples 1 to 6, Comparative Examples 1 to 5] In each example and comparative example, a continuous mixer / reactor was used, consisting of a paddle with an outer jacket for passing a heat (cool) medium, a cross section shaped like two overlapping circles, and a rotating shaft with a paddle attached. While each of the two rotating shafts with paddles attached was rotating at 150 rpm, trioxane (A), 1,3-dioxolane as a cyclic acetal compound (B), and the aliphatic glycidyl ether compound (C) shown in Tables 1 and 2 were added in the proportions and amounts shown in Tables 1 and 2, respectively. Furthermore, to obtain a copolymer with a predetermined melt flow rate, a predetermined amount of methylal was continuously added as a molecular weight modifier. A homogeneous mixture of boron trifluoride gas catalyst, mixed to a concentration of 0.005% by mass, calculated as boron trifluoride, relative to the trioxane, was continuously added to carry out bulk polymerization. The reaction product discharged from the polymerization reactor was quickly passed through a crusher and added to an aqueous solution containing 0.1% by mass of triethylamine at 80°C to deactivate the catalyst. After separation, washing, and drying, a crude polyacetal copolymer was obtained. The amount of methylal used as a molecular weight modifier was set to satisfy (b + c + d) / a = 1.5 to 7 μmol / g, where a is the total mass (g) of trioxane (A), cyclic acetal compound (B), and aliphatic glycidyl ether compound (C), b is the number of moles of methylal, and c and d are the total moles of water and methanol contained in components (A), (B), and (C), respectively.

[0046] Next, 4 parts by mass of a 5% by mass aqueous solution of triethylamine and 0.03 parts by mass of pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] were added to 100 parts by mass of the obtained crude polyacetal copolymer, and the mixture was melt-kneaded at 210°C in a twin-screw extruder to remove unstable portions.

[0047] To 100 parts by mass of the polyacetal copolymer obtained by the above method, 0.3 parts by mass of pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 0.15 parts by mass of melamine were further added as stabilizers, and the mixture was melt-kneaded at 210°C in a twin-screw extruder to obtain pelletized polyacetal resin.

[0048] [Fatigue test] In each example and comparative example, the obtained polyacetal resin pellets were used to mold gears in the shape of standard spur gears under the following molding conditions: module (m) = 1.0, number of teeth: 54, face width: 10 mm. (Gear molding conditions) Molding machine: SE100D, manufactured by Sumitomo Heavy Industries, Ltd. Molding temperature: 200℃ Mold temperature: 80℃ Injection speed: 20mm / s Next, for each example and comparative example, the molded gear was used to measure the number of rotations until the gear broke under the following evaluation conditions with grease applied. The measurement results are shown in Tables 1 and 2. During gear rotation, a load was applied under the load conditions shown in Tables 1 and 2. (Evaluation conditions) Testing machine: Small gear fatigue testing machine (Ono Sokki Co., Ltd.) Evaluation temperature: Room temperature Evaluation rotation speed: 300 rpm Grease: DuPont Toray Specialty Materials Co., Ltd., Molycoat EM-30L

[0049] [Table 1]

[0050] [Table 2]

[0051] Tables 1 and 2 show that Examples 1 to 6 all had a higher number of revolutions until breakage and were superior in fatigue resistance than Comparative Examples 1 to 4. Comparative Examples 1 to 4 had either too much or too little (or no) of the structural units derived from component (B) or the structural units derived from component (C), and therefore had poor fatigue resistance.

Claims

1. A method for improving fatigue resistance of a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin, comprising: The method for improving the fatigue resistance of a polyacetal resin gear includes using, as the polyacetal resin, a polyacetal resin obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule, wherein the cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % in the total of the components (A) to (C), and determining whether the following fatigue condition is satisfied: [Fatigue conditions] The polyacetal resin was used to mold a gear in the shape of a standard spur gear with a module (m) of 1.0, 54 teeth, and a face width of 10 mm. The molded gear was used to test the following evaluation conditions with grease applied: the number of rotations until the gear broke when a torque of 13 N m was applied was 67,500 to 1,510,000, when a torque of 15 N m was applied, was 198,000 to 610,000, and when a torque of 17 N m was applied was 41,000 to 58,000. (Evaluation conditions) Testing machine: Small gear fatigue testing machine (Ono Sokki Co., Ltd.) Evaluation temperature: Room temperature Evaluation rotation speed: 300 rpm Grease: Molycoat EM-30L, manufactured by DuPont Toray Specialty Materials Co., Ltd.

2. A method for improving fatigue resistance of a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin, comprising: The polyacetal resin is a polyacetal resin obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule, wherein the cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % in the total of the components (A) to (C); and A method for improving the fatigue resistance of a polyacetal resin gear, comprising providing a gear using a polyacetal resin that satisfies the following fatigue conditions (with the exception of the case where the polyacetal resin is obtained by copolymerizing 100 parts by mass of trioxane, 1.23 parts by mass of 1,3-dioxolane, and 1.0 part by mass of butyl glycidyl ether using a cationic polymerization catalyst, or the case where the polyacetal resin is obtained by copolymerizing 100 parts by mass of trioxane, 1.23 parts by mass of 1,3-dioxolane, and 0.3 parts by mass of butyl glycidyl ether using a cationic polymerization catalyst): [Fatigue conditions] The polyacetal resin was used to mold a gear in the shape of a standard spur gear with a module (m) of 1.0, 54 teeth, and a face width of 10 mm. The molded gear was used to test the following evaluation conditions with grease applied: the number of rotations until the gear broke when a torque of 13 N m was applied was 67,500 to 1,510,000, when a torque of 15 N m was applied, was 198,000 to 610,000, and when a torque of 17 N m was applied was 41,000 to 58,000. (Evaluation conditions) Testing machine: Small gear fatigue testing machine (Ono Sokki Co., Ltd.) Evaluation temperature: Room temperature Evaluation rotation speed: 300 rpm Grease: Molycoat EM-30L, manufactured by DuPont Toray Specialty Materials Co., Ltd.

3. A method for improving fatigue resistance of a polyacetal resin gear made of a polyacetal resin or a resin composition containing a polyacetal resin, comprising: The polyacetal resin is a polyacetal resin obtained by copolymerizing trioxane (A), a cyclic acetal compound (B), and an aliphatic glycidyl ether compound (C) having one glycidyloxy group per molecule, wherein the cyclic acetal compound (B) accounts for 0.5 to 2.5 mol % and the aliphatic glycidyl ether compound (C) accounts for 0.02 to 0.7 mol % in the total of the components (A) to (C); and The aliphatic glycidyl ether compound (C) is 2-ethylhexyl glycidyl ether; and A method for improving the fatigue resistance of a gear made of polyacetal resin, comprising providing a gear using a polyacetal resin that satisfies the following fatigue conditions: [Fatigue conditions] The polyacetal resin was used to mold a gear in the shape of a standard spur gear with a module (m) of 1.0, 54 teeth, and a face width of 10 mm. The molded gear was used to test the following evaluation conditions with grease applied: the number of rotations until the gear broke when a torque of 13 N m was applied was 67,500 to 1,510,000, when a torque of 15 N m was applied, was 198,000 to 610,000, and when a torque of 17 N m was applied was 41,000 to 58,000. (Evaluation conditions) Testing machine: Small gear fatigue testing machine (Ono Sokki Co., Ltd.) Evaluation temperature: Room temperature Evaluation rotation speed: 300 rpm Grease: Molycoat EM-30L, manufactured by DuPont Toray Specialty Materials Co., Ltd.

Images